1. Field of the Invention
The present invention relates to a technique for controlling opening and closing of a valve, and particularly relates to a valve control system, a valve control apparatus, and a valve control method for controlling opening and closing of a valve with the use of an actuator such as a pulse motor, etc.
2. Description of the Related Art
Electrically-driven valves are used for various purposes. For example, a temperature-sensitive expansion valve (thermal expansion valve) is used for controlling the flow rate of a refrigerant in Unexamined Japanese Patent Application KOKAI Publication No. S56-44569.
Further, an improved method for driving the temperature-sensitive expansion valve disclosed in Unexamined Japanese Patent Application KOKAI Publication No. S56-44569 is disclosed in Examined Japanese Patent Application KOKOKU Publication No. H6-86961.
The driving method disclosed in Examined Japanese Patent Application KOKOKU Publication No. H6-86961 is an improvement for PID (Proportional Integral Differential) control. According to this driving method, the opening degree of an electrically-driven expansion valve is controlled in accordance with the sum of a first electric signal which is the product of an electric signal indicative of a temperature difference between a degree of superheat of a refrigerant at the exit of an evaporator and an aimed degree of superheat and a first proportionality constant, a second electric signal which is the product of an electric signal indicative of a value obtained by integrating that temperature difference with respect to time and a second proportionality constant, and a third electric signal which is the product of an electric signal indicative of a value obtained by differentiating that temperature difference with respect to time and a third proportionality constant. This driving method can achieve a better control on the degree of superheat than in a case where a temperature-sensitive expansion valve is used.